This invention relates to the radiant energy imaging of humans to detect weapons, explosives, contraband, and other security threats hidden under the clothing.
Criminals and terrorists frequently conceal security threats under their clothing, such as handguns, knifes, explosives and illicit drugs. These security threats must be detected on persons entering security controlled areas, such as prisons, airports, government buildings, nuclear power plants, military bases, and the like. Searching individuals by hand is time consuming, often ineffective, and objectionable to both the person being screened and the security officer performing the screening. Electronic imaging systems became commercially available in the 1990s to facilitate this screening process. These include the model SECURE 1000, sold by Rapiscan Security Products; model SmartCheck, sold by American Science and Engineering; and model ProVision, sold by L3 Communications. These electronic imaging systems are commonly referred to as “body scanners.”
Body scanners operate by exposing the person being screened to radiant energy, such as millimeter waves or x-rays. A portion of the radiant energy interacts with the person, their clothing, and any concealed objects they may be carrying. This interaction modulates the radiant energy that is reflected from, scattered by, or transmitted through the person. This reflected, scattered or transmitted radiant energy is collected by sensitive detectors, and the resulting electronic signals are routed to a digital computer. Software operating in the digital computer converts the electronic signals into digitally represented images of the person's body. In these images the clothing is essentially transparent, allowing the security officer to visualize any objects that are concealed under the clothing.
Prior art body scanners are capable of detecting a wide range of objects concealed under the clothing. However, the human body is complex in shape, and the type of clothing worn by people is diverse and unpredictable. This results in areas of the body where prior art body scanners are ineffective in detecting objects. Of particular concern are the sides of the body and the shoes, where certain types of threats are likely to be missed by prior art systems. Further, prior art body scanners are physically large, and therefore difficult to incorporate into space-limited security checkpoints, such as airports.
FIG. 1 depicts the operation of one type of prior art x-ray body scanner, such as described in U.S. Pat. Nos. Re. 28,544; 5,181,234; and 6,665,373. An x-ray source 10 produces an x-ray beam 11 that is directed at the examined person 12. The cross-section of the x-ray beam 11 is typically about 6 mm×6 mm where it strikes the examined person 12. One of three outcomes will be experienced by each individual x-ray in the x-ray beam 11. First, the x-ray may interact with the body tissue through the photoelectric effect and be annihilated. Second, the x-ray may interact with the body tissue through Compton scattering, which alters its direction of propagation. X-rays that are scattered in the forward direction, that is, into the body, are not useful and are ignored. However, x-rays that scatter in the reverse or backward direction, called backscatter x-rays 20, carry useful information. Third, an individual x-ray may pass completely through the examined person 12, or pass around the examined person 12, without interacting. These are called transmitted x-rays 15, and also carry useful information.
The backscatter x-rays 20 are detected by backscatter detector 30 to produce an electronic backscatter signal 35. Likewise, the transmitted x-rays are detected by vertical transmission detector 18 and floor transmission detector 19, thereby generating an electronic transmission signal 36. Both the backscatter signal 35 and the transmission signal 36 are routed into digital computer 60. As thus described, the instantaneous value of the backscatter signal 35 is a measurement of the backscattering properties of the examined person 12 at the location on the body where the x-ray beam 11 is incident. In a similar fashion, the instantaneous value of the transmission signal 36 is a measurement of the transmission properties of the examined person 12 at the location on the body where the x-ray beam 11 transits through the body. These instantaneous values of the signals are recorded by digital computer 60. Subsequent measurements are made on all other locations on the body of the examined person 12 by redirecting the x-ray beam 11 to those locations, a technique known in the art as a “flying spot.” As disclosed in the above referenced U.S. patents, this flying spot scanning may be accomplished by a rotating chopper assembly for sweeping the x-ray beam in a horizontal arc, in conjunction with a vertical displacement or rotation of the x-ray source 10. These apparatus and methods for steering the x-ray beam 11 are well known to those skilled in the art. A control signal 40 synchronizes the data collection of digital computer 60, allowing it to format the measurements into electronic images. Specifically, the series of measurements appearing in the backscatter signal 35 is formatted into a backscatter image 50. Likewise, the series of measurements appearing in the transmission signal 36 is formatted into a transmission image 70. Accordingly, the backscatter image 50 is representative of the modulation produced by x-ray beam 11 being backscattered by the examined person 12. Similarly, the transmission image 70 is representative of the modulation produced by the x-ray beam 11 being transmitted through the examined person 12.
As thus described, the prior art body scanner depicted in FIG. 1 acquires a backscatter image 50 and a transmission image 70 from a single viewpoint on the anterior or front side of the examined person. That is, the depicted body scanner would be said to be a “single-view, dual mode” system, acquiring a front-backscatter image and a front-transmission image. An important aspect of the prior art body scanner depicted in FIG. 1, as it pertains to the present Invention, is that the x-ray source 10 is physically positioned between and/or behind the x-ray detectors 30. An attribute of this prior art embodiment is that both the transmission image 70 and the backscatter image 50 have a complete field-of-view. That is, the entire examined person 12 appears in the images 50 70 from head to toe, with no regions missing from the image acquisition. In addition, this embodiment operates with stationary detectors 18 19, thereby avoiding the problem of moving sensitive electronics during the image acquisition.
A limitation of the prior art embodiment of FIG. 1 is that the examined person 12 must turn their body to obtain a rear scan. Another limitation of this embodiment is that the shoes are examined with the same apparatus that scans the whole body. That is, the shoes are portrayed in the same backscatter image 50 and transmission image 70 as the front of the body, and not otherwise examined. However, these images of the prior art are not sufficient to detect security threats in the shoes for two important reasons. First, the backscatter image 50 is only a view from an angle generally above and in front of the shoes, and cannot visualize concealed objects hidden underneath the foot, or concealed within the sole and heal of the shoes. Second, the spatial resolution of the transmission x-ray image 70 is too low to adequately inspect the shoes. As known in the art, the spatial resolution of a flying spot imaging system is determined by the cross-section of the x-ray beam 11 where it strikes the examined person 12. For prior art body scanners this is typically about 6 mm, which is insufficient to resolve the bones in the feet of the examined person 12. In turn, the insufficiently resolved bones in the feet produce an image clutter that drastically interferes with the visualization of concealed objects. Put in other words, the task of the security operator viewing the image is to discriminate between the normal anatomy of the feet and non-anatomic objects contained in the shoes. If the operator cannot clearly identify the complex pattern of bones in the feet, they cannot distinguish these bones from hidden objects.
These limitations and attributes of the prior art depicted in FIG. 1 can be compared with a second embodiment of the prior art. FIG. 2 is a depiction of another prior art body scanner geometry, in accordance with U.S. Patent Application 2009/0116617. This consists of a front scanner 8 and a rear scanner 7, which are essentially identical in physical structure. This front-back symmetry allows both backscatter and transmission images to be acquired from both the anterior and posterior viewpoints. For the posterior or rear scanning cycle, rear carriage assembly 80 emits an x-ray beam 11 horizontally, striking examined person 12. As previously explained, a portion of the x-ray beam 11 will become backscatter x-rays 20, and a portion will become transmitted x-rays 15. The backscatter x-rays 20 are detected by a rear-upper detector 31 and a rear-lower detector 32, and used to create a rear-backscatter image. The transmitted x-rays 15 are detected by a front-upper detector 33, and used to create a rear-transmission image. As known in the art and disclosed in the above referenced document, the x-ray beam 11 is scanned horizontally in an arc to acquire one line in both the backscatter and transmission images. To complete the vertical component of the raster scan, rear carriage 80 and front carriage 81 move vertically in synchronization, maintaining alignment of the transmitted x-rays 15 with the front-upper detector 33.
Because the front scanner 8 and the rear scanner 7 are identical in physical structure, the above operation can be repeated in a mirror image fashion. That is, a front-backscatter image is acquired by emitting an x-ray beam from the front carriage 81, and detecting backscatter x-rays with front-upper detector 33 and front-lower detector 34. Likewise, a front-transmission image is simultaneously acquired by detecting transmitted x-rays with upper-rear detector 31. Accordingly, the prior art body scanner depicted in FIG. 2 acquires both transmission and backscatter images from both the front (anterior to the examined person 12) and rear (posterior to the examined person 12), and therefore would generally be called a “dual-view dual mode” system. However, this prior art system does not fully meet this classification, since transmission images from both the front and rear cannot be obtained of the lower legs and feet. The lowermost location on the examined person 12 that will appear in the transmission image is limited by the physical size of the front carriage 80 and the rear carriage 81. That is, when the carriages 80 81 are in their lowermost position, their respective x-ray beams will be some distance above the ground. Tilting the x-ray beams toward the ground allows for acquisition of complete backscatter images, including the feet. However, no detector is present that is capable of detecting x-rays that are transmitted through the lower legs and feet. In a practical embodiment the lower 12″ to 18″ of the legs will not appear in the images. As before, an important aspect of the prior art body scanner depicted in FIG. 2, as it pertains to the present Invention, is that the x-ray source is physically positioned between and/or behind the x-ray detectors.
As thus described, prior art body scanners are limited in their ability to search persons entering security controlled areas by the number of images they can acquire of each person. At most, prior art systems acquire dual-view dual-mode images, with the transmission images being only partial images. This results in blind areas in detection on the body of the person being screened. Further, the prior art apparatus has a physically large footprint, and therefore is not able to be installed in security checkpoints where floor space is limited. For instance, the apparatus in FIG. 2 is approximately nine feet wide, with three feet for the width of the rear scanner 7, three feet for the positioning of the examined person 12, and three feet for the width of the front scanner 8. Further, prior art body scanners can scan a person without the person turning their body, as depicted in FIG. 2, or they can create complete transmission images of the entire person's body, as depicted in FIG. 1. However, no prior art body scanner can do both in a single apparatus.